Syrups containing a mixture of glucose and fructose are widely used in industry for their sweet taste and their low tendency to crystallize. Such syrups are produced from glucose syrups using a xylose isomerase that exhibits glucose isomerase activity to catalyze isomerization of glucose to fructose. It is important for the economy of this enzymatic process that the enzyme costs be low and that there be a negligible formation of by-products requiring removal before the isomerized syrup can be used.
The production costs of microbially derived xylose isomerase depends largely on the cost of the fermentation substrate and the yield of enzyme from the cultivation. The most important enzyme characteristics for economic usefulness in the glucose isomeration process are enzyme purification yields and capability in the recovered enzyme to be converted to an immobilized enzyme product exhibiting satisfactory physical and enzyme chemical properties, including notably good stability at high temperatures, resistance towards physical pressure and a high half life.
No one of the xylose isomerases, heretofore known including, of course, the glucose isomerase products commercially heretofore employed are truly superior in all desired enzyme characteristics. The art has accepted compromise products.
As might be expected, a great amount of research has been carried out in efforts to discover a superior xylose isomerase from one microbiological source or another. This research is reflected in the patent literature. A large number of patents are directed to different microbial sources of xylose isomerase. Many bacteria, e.g., species belonging to the genera Streptomyces, Actinoplanes, Bacillus, and Flavobacterium, and fungi e.g., species belonging to the class Basidiomycetes, have been identified in the patent literature as xylose isomerase producing microorganisms. Also, just to choose some examples, from the bacterial genus of interest to this invention, namely, Streptomyces the following have been described in the patent literature as xylose isomerase producing microorganisms.
______________________________________ U.S. Pat. No. 3,616,221 S. flavovirens S. albus S. achromogenus S. wedmorensis S. echinatus U.S. Pat. No. 3,622,463 S. olivochromogenes S. venezuelae U.S. Pat. No. 4,351,903 S. griseoflavus U.S. Pat. No. 4,137,126 S. glaucescens U.S. Pat. No. 3,625,828 S. olivaceus Hungary Patent No. 12,415 S. galbus S. niveus S. gracilis S. platensis S. matensis Germany Patent No. 2,417,642 S. violaceoniger U.S. Pat. No. 4,399,222 S. acidodurans Japan Patent No. 69,28,473 S. phaeochromogenes S. californicus S. fradiae S. vanaceus S. rosechromogenes S. virginiae S. olivaceus ______________________________________
Thus, the object of this invention is to provide a superior microbial xylose isomerase, namely, one which can be produced with an overall good compromise in regard to fermentation yield, purification yield and ability of the produced xylose isomerase to be converted to an immobilized product with satisfactory physical and enzyme chemical properties.